The anaerobic digestion of organic liquid waste streams has been a fundamental part of waste treatment for hundreds of years. Municipal and industrial wastes have been treated utilizing anaerobic digestion techniques for over 100 years in the United States, and within the last thirty years, anaerobic digestion of higher strength animal wastes has also become an accepted practice. However, a limitation of the bacterial-based anaerobic digestion process has been the inability of anaerobic bacteria to grow outside the parameter of a narrow pH range.
Anaerobic digestion comprises two main classes of anaerobic bacteria: acid forming bacteria (acid formers) and methanogenic bacteria (methane formers). The acid forming bacteria perform best at a pH of about 6.0 to about 7.0 and the methanogenic bacteria perform best at a pH of about 6.5 to about 8.0. These narrow pH ranges preclude effectively utilizing anaerobic digestion waste treatment technology for the treatment of high-strength organic, liquid waste streams having a pH below about 6.5 or above about 8.0.
A high-strength organic liquid waste stream typically has a solids content of about 5% to about 40%. Acidic high-strength organic liquid wastes have a pH of less than about 5.0. Examples of such wastes include acidic cheese whey, with a pH of about 3.5, and the rapidly growing wastes from ethanol plants, with a pH of about 3.5 to about 4.0 and a solids content of about 30% to about 35%. When anaerobic digestion has been attempted with acidic high-strength organic liquid wastes in mixed digesters, the traditional response has been to adjust the pH of the acidic wastes to about 7 with either the addition of expensive chemical pH adjusters or the blending of alkaline waste streams with the acidic wastes. Alkaline high-strength organic liquid wastes have a pH greater than about 8.0. Examples of such wastes include the glycerin by-product waste from biodiesel plants that convert animal oils or vegetable oils into biodiesel. Glycerin typically has a pH of about 12 to about 14 and a high solids content of about 20% to about 35%. When anaerobic digestion has been attempted with alkaline high-strength organic liquid wastes in mixed digesters, the traditional response has been the addition of expensive, corrosive acids, such as sulfuric or citric acids, to lower the pH of the entire digester prior to the anaerobic biodegradation so that the influent pH of the waste stream is continuously adjusted to a pH of about 7.